Is this some hard limit or is this just because a regular motherboard would just not fit more? I would imagine that there are options to get more to fit, like watercooling the cards so they take up one slot instead of two or utilizing a PCIe expander (which begs the question, how many lanes does each card need for optimum mining performance, x4.x8,x16?).

Surely I'm not the only one thinking of how to get 20 cards running in a single system?

8 GPU's in windows16 In linux... i think...this is a limmitation of the open CL driver

so 4 6990 in windows8 in linux

as for how much PCIe bus you need.

PCIe 1x is more then enough.very little data is trasfered across for the GPU to process.

Do you perhaps know whether this is a hard limit or a limit of convenience (i.e. some kernel or memory addressing limit / they figured 4/8 would be enough)?

not sure.however in older drivers the windows limit was 4 GPUs and just recently increased it to 8.i wish it was 256... then for each motherboard you could max it out using PCIE splittersbut unfortuatly you can't.

256 GPUs.... I think you've lost sight of another factor... There is a reason I only have 2 6990s in my rig, cost.

but not everybody has your limited wallet size...think of the money yuo would save...i.e running windows...for every 8 GPUs you need another motherboard, CPU, ram, harddrive.that is the one shot cost...long term cost is the power needed to supply each MB, CPU .....if you could get a 256 GPUs running on a single machine it would save $

there are PCIe splitters that will take a 16x slot and split it to 2 8x slots.or 16x to 4 4x slots.i know of a few boards that have 7 slots. one beeing Asus P8P67 Work station. has 4 16x slots and 3 1x slots.that is what i'm using... have 4 of them running.